Several antivirus vendors have taken the open-source Chromium browser and created derivatives that they claim are more privacy-friendly and secure. Yet, at least two of them were recently found to have serious flaws that don’t exist in Chromium.

The latest example is the Avast SafeZone browser, internally known as Avastium, which is installed with the paid versions of Avast’s antivirus and security suites. Google Project Zero researcher Tavis Ormandy found a vulnerability that could allow an attacker to take control of Avastium when opening an attacker-controlled URL in any other locally installed browser.

By exploiting the flaw, an attacker could remotely read “files, cookies, passwords, everything,” Ormandy said in a report that he sent to Avast in December and which he made public Wednesday. “He can even take control of authenticated sessions and read email, interact with online banking, etc.”

Ormandy created a Web-based proof of concept exploit that can list the contents of the computer’s C:\ drive, but an attacker could easily extend it to have any potentially interesting files sent back to him.

According to the Google researcher, Avast opens a Web accessible RPC service on the local computer that listens on port 27275. A malicious website opened in any browser can therefore send commands to this service by forcing the browser to make requests to http://localhost:27275/command.

While most of the available commands are not particularly dangerous, there is one called SWITCH_TO_SAFEZONE that can be used to open a URL in Avastium. And not just any URL like http:// or https:// ones, but also local or internal URL schemes like file:/// or chrome://.

That’s because, for some reason, Avast has removed what Ormandy calls a “critical security check” that prevents non-Web-related URL schemes from being opened from the command line. This protection, which exists in the original Chromium, was not present in Avastium, making it possible for an attacker to ultimately construct a payload that can read local files.

After Ormandy reported the flaw on Dec. 18, Avast deployed a temporary fix that broke the attack chain. The company provided a complete fix Wednesday as part of Avast version 2016.11.1.2253.

This week Ormandy also disclosed a critical vulnerability in Chromodo, another Chromium-based browser that’s distributed by security firm Comodo as part of its Internet Security suite. That vulnerability stemmed from the fact that Chromodo disabled one of the most critical browser security mechanisms, the Same Origin Policy.

Avast and Comodo are not the only security vendors who have created so-called “safe” browsers based on Chromium and are shipping it with their products. If Ormandy continues to investigate them, it will be interesting to see if he finds additional examples of serious flaws that were introduced in such browsers and are not present in Chromium.

Joxean Koret, a security researcher who has found vulnerabilities in antivirus products in the past, advised people on Twitter not to use the browsers provided by antivirus vendors. “I’ve analyzed 3. All broken,” he said.

“Selling antivirus doesn’t qualify you to fork chromium, you’re going to screw it up,” Ormandy said in a Twitter message earlier this week.

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